The present invention relates to a piezoelectric actuated device for manually or automatically transferring very small volumes of fluid. Transferring fluids from one container to another is one of the most common tasks performed in a typical chemical or biological laboratory. For example, various chemicals from different containers may have to be mixed together and then the mixture may have to be divided out equally or `aliquoted` into other containers or onto microscope slides or some other substrates. Currently transferring of fluids is done either by hand using manual pipettors or automatically using an automated robotic pipetting instruments. Motivated by the high costs of chemical reagents there is a trend in chemical and biological laboratories to perform experiments using smaller and smaller fluid samples.
Prior to the 1990's fluid transfer was done in laboratories by hand using glass tubes called pipettes. Samples were aspirated by sucking on the end of the pipette by mouth like a straw and then sealing the end with a finger. Fluid samples were then dispensed or aliquoted by briefly unsealing and then resealing the pipette with the finger allowing a small volume of fluid to flow out. This method required a fair amount of dexterity and had some serious drawbacks in terms of ergonomics, precision and health risks. During the same time period glass capillaries were also used for transferring smaller volumes of fluid with greater precision. These techniques for manually transferring fluid samples were made obsolete in the early 1990's by the introduction of the manual pipette as described, for example, in U.S. Pat. Nos. 3,613,952, 3,766,785, 3,827,305 and 3,991,617, with ejectable, disposable plastic tips. Currently the manual pipette is a ubiquitous and indispensable tool for transferring fluid samples in the modern laboratory.
Although manual pipetting is a big improvement over earlier techniques it is tedious for the human operator and can cause repetitive motion injuries as well as being prone to human error. Currently, a great variety of automated pipetting instruments are commercially available which address these specific drawbacks. These instruments, as advanced by Tecan AG of Hombrechtikon Switzerland, typically use a Cartesian X,Y,Z robot to move a pipetting head among various aspirating, dispensing and washing stations. The various types of instruments currently available differ mainly in the mechanisms used to acquire and then dispense fluid samples. The most common mechanism for fluid transfer is the syringe pump as used in instruments manufactured by Tecan, Hamilton Company, Cavro Scientific Instruments, Robbins Scientific, Qiagen and Tomtec among others. In most cases the syringe pumps and pipette tips are connected via long flexible tubing, however, in the instruments supplied by Tomtec and Robbins Scientific the syringe pumps are mounted directly to the back of the pipette tips. The smallest volume that can be accurately transferred using syringe pumps is approximately 0.1 microliters. U.S. Pat. Nos. 5,743,960 and 5,741,554 describe an instrument which combines a syringe pump with a solenoid valve allowing drops as small as 0.1 microliters to be ejected onto a substrate for non-contact printing or arraying applications. By contrast, the drops that are dispensed using the present invention are a thousand times smaller.
Another method for transferring small volumes of fluid for arraying applications is the use of pins as described in U.S. Pat. No. 5,807,522. Instruments using pins for fluid transfer are used by Synteni, among other companies, to generate DNA arrays and are commercially available, for example, from BioRobotics and GeneMachines. Using pins is a simple, robust and practical means for fluid transfer but it suffers from some limitations. First of all it is slow. The pins have to stop at each spot and then wait for over a second for capillary action to transfer the fluid onto the substrate. By contract, the piezoelectric based dispensing of the present invention is almost a thousand times faster. Pin based fluid transfer is sensitive to the wetting properties of the substrate. Also it can damage some substrates like Nylon membranes for example. These are not concerns for the piezoelectric dispensing which is non-contact. Pins generate relatively low density, poor quality arrays with approximately 50% variability in spot size. By contrast, piezoelectric dispensers generate arrays with almost an order of magnitude higher density and better than 3% spot size variability. Finally, pins are limited to acquiring and dispensing a fixed volume of fluid. Piezoelectric dispensers have thousands of times higher dynamic range. Sub nanoliter to tens of microliter volumes can be aspirated and subsequently dispensed in volumes ranging from 100 picoliters to several microliters per second.
A number of companies including Microdrop, Packard Instruments and GeSiM, use piezoelectric fluid dispensing devices to dispense drops of fluid with volumes on the order of 100 picoliters. Originally piezoelectric dispensing technology was used for "drop-on-demand" "ink-jet" printing. These devices as described in U.S. Pat. No. 2,512,743 have a fluid filled chamber with an inlet at one end and a nozzle on the other. A piezoelectric element induces an acoustic wave in the fluid causing a drop to be ejected from the nozzle. For printing applications ink is supplied to the back end of the piezoelectric fluid dispenser from a reservoir. For fluid transfer applications fluid is drawn up through the nozzle. The instruments supplied by Microdrop, Packard Instruments and GeSiM all use syringe pumps to aspirate fluids up through the nozzle of the piezoelectric devices. Syringe pumps impose several serious limitations on current piezoelectric based pipetting instruments. It turns out that properly aspirating samples prior to dispensing is one of the most critical considerations for reliable operation of piezoelectric fluid dispensers. Syringe pumps can sometimes aspirate air bubbles and small particles that can adversely affect the dispensing characteristics. Also, variations in back pressure in the flexible tubing leading to the syringe pumps causes the drop-on-demand dispensing properties to change. Packard Instruments uses a closed loop pressure controller to actively regulate this back pressure. This system adds to the cost and complexity of the instrument. Additionally the syringe pumps themselves add significantly to the cost, size and complexity of the instrument and they limit the smallest fluid volumes that can be transferred to around 0.5 microliter. By contrast, with the present invention, volumes over 100 times smaller can be aspirated. Most importantly, aspirating is much more uniform and precise making dispensing more reliable and repeatable. In the present invention, the dispensers are opened to ambient pressure so the back pressure always remains uniform. Additionally the novel sensing technology of the present invention detects almost instantly if there is a problem with the dispensers, e.g. if they are clogged, empty or attempting to aspirate from an empty well.
The present invention provides improvements to the cylindrical piezoelectric fluid dispenser described in U.S. Pat. No. 3,840,758. These improvements enable bi-directional operation of the device, sensing of the operational state and thermal control. As a summary of some related devices. Humberstone in U.S. Pat. No. 5,518,179 describes a bi-directional device with a piezoelectric driven, thin, perforated membrane capable of drawing a fluid up through the perforations as well as dispensing fluid out through them. Beckman et al. in U.S. Pat. No. 4,432,699 describes a peristaltic piezoelectric pump with an internal load sensor. Hayes in U.S. Pat. No. 5,622,897 describes a process of manufacturing a drop-on-demand ink-jet print head having n-type and p-type thermoelectric carriers.